1. Field of the Invention
The present invention relates to a device for recovering a carrier, and more particularly, to a device for recovering a carrier of a digital TV receiver.
2. Background of the Related Art
The VSB (Vestigial SideBand) system, employed as a broadcasting standard in Korea and the USA presently, is designed to transmit a broadcasting signal by using a frequency assigned to existing analog TV broadcasting. However, for minimizing influence to the existing analog TV broadcasting, an intensity of a DTV signal is very low compared to an intensity of an analog TV signal. Of course, the standard is determined such that there is no problem in reception of the DTV signal even if the intensity of the signal is low by using a variety of coding systems in the DTV signal and channel equalizers for reduction of influence from a noise. However, if the signal can not be received properly if a transmission channel condition is very poor. In general, because the DTV transmission system has a merit in that you can watch a picture having no noise at all as noise occurred on the transmission channel is removed perfectly in reception of the broadcasting signal, but has a demerit in that you can not watch the picture at all if you can not recover the transmission signal fully, it is required that the receiver can receive all signals even if the signals have passed through any level of poor transmission channel.
In the meantime, noticeable features of the VSB transmission system suggested from GA compared to other DTV transmission system are the pilot signal, the data segment synchronizing signal, the data segment synchronizing signal, and the field synchronizing signal. Those signals are transmitted from a transmitting part together with the DTV signal for improving characteristics of carrier recovery, timing recovery, and the like.
FPLL (Frequency Phase Locked Loop) is used as the most general algorithm for recovering the carrier, fabrication of which circuit is simple and has an excellent performance to use widely. That is, a carrier recovering part of the FPLL demodulates I, Q signals of a passband from an A/D converter into I, Q signals of a baseband, and locks a frequency and a phase. To do this, the FPLL conducts an FPLL process for removing a frequency difference between a carrier component of a received signal, and a component of reference carrier of the receiver itself, and a PLL (Phase Locked Loop) process for removing a phase error between the two carrier signals having the frequency difference removed therefrom.
However, the phase difference occurs continuously during operation of the receiver too, it is required to carry out the PLL continuously for correcting the phase error during operation of the receiver.
Moreover, though a deviation of the Q signal after an average of DC components of the Q signal is made to be ‘0’ is small if an amplitude of the carrier of the received signal is significantly greater than amplitudes of noises, a deviation of the Q signal is significantly great owing to an influence from the noise though the average of the DC components of the Q signal is ‘0’ if the amplitude of the carrier of the received signal is similar to, or smaller than the amplitude of the noise, to cause frequency jitter, and a poor performance of the receiver due to the Q signal deviation.